1. Field of the Invention
The invention relates to communication and navigation systems. More particularly, it relates to mobile communications systems with satellite and terrestrial navigation features.
2. Description of the Related Art
Satellite-based navigation systems, such as the Global Positioning System (GPS), allow for accurate navigation almost anywhere in the world. A constellation of GPS satellites surrounds the earth in three different planes with the satellites orbiting the earth every 12 hours. The orbit of each satellite is closely monitored and its position is precisely known. Each GPS satellite transmits navigation signals, each signal being modulated with at least one pseudo-random noise (PN) code, or a portion of a PN code, unique to the satellite. Each GPS satellite uses a very stable frequency reference for generating the satellite""s signals. Also modulated onto the navigation signals is a satellite data message (SDM) with a bit rate of 50 bits/second, which is much slower than the chipping rate of the PN code. The signal structure of the GPS satellites is described in further detail in Spilker, Jr., J. J. xe2x80x9cGPS Signal Structure and Performance Characteristics,xe2x80x9d Global Positioning System, Papers Published in Navigation, The Institute of Navigation, Washington, D.C., 1980, which is incorporated by reference herein for all purposes.
Using a GPS receiver with an unobstructed view of the sky a person can determine his or her position within a few meters. In order to determine its position in three dimensions, and to determine time precisely, the receiver must receive navigation signals from at least four satellites, as shown in FIG. 1. Here, a mobile unit (MU) can determine its position (x, y, z) and time by determining its distance from four sources, in this case four GPS satellites (10-1, 10-2, 10-3 and 10-4), at known positions ((x1, y1, z1), (x2, y2, z2), (x3, y3, z3), (x4, y4, z4)). A conventional GPS receiver is shown in FIG. 2. It includes an antenna 20, a front end 30 for converting the received signals into digital signals in a form suitable for processing,.an array of delay lock loop (DLL) circuits 40 and a Kalman filter estimator 50 for estimating the receiver""s position. Each DLL circuit outputs a pseudo-range measurement (PR1 through PRn, respectively) to a summation unit (units 511 through 51n) in the Kalman filter estimator 50. Each summation unit combines the latest pseudo-range measurement from the DLL with the Kalman filter""s estimate of the receiver""s position x(t) and provides the combined signal to a vector estimation unit 52. The vector estimation unit 52 uses the outputs of the summation unit to update the Kalman filter""s estimate of the receiver""s position and outputs a position vector x{circumflex over ( )}(t).
A schematic diagram of each of the delay lock circuits (DLL1 through DLLn) is shown in FIG. 3, shown here as a coherent DLL, although a non-coherent DLL also can be used. Each DLL circuit includes a local PN code generator 65 that generates the same PN codes broadcast by the GPS satellite the DLL circuit is tracking. The conventional receiver also includes an early correlator 60E and a late correlator 60L that correlate the received signal (i.e., the satellite signal A(t) at t=xcfx84, plus noise n(t)) with an early and late versions of the locally generated PN code S0(t). As shown in FIG. 3 the early version of the PN code is advanced by half a PN chip period T (i.e., +T/2) and the late version is delayed by half a PN chip period (i.e., xe2x88x92T/2). The early and late correlation signals are each filtered by low pass filters 61E and 61L, respectively. The filtered signals are combined using combiner 62 and the resultant signal is filtered by loop filter 63. The output of loop filter 63 is a control signal that controls a number controlled oscillator (NCO) 64. The NCO generates a number, based on the control signal from loop filter 63, that causes the PN generator 65 to output its PN sequence either faster or slower depending on which of the early and late correlators outputs a stronger signal. The DLL circuit tracks the code in the received signal and a time measurement, called a pseudo-range measurement, is made. The measured pseudo-range is a measure of the signal propagation time between the satellite and the receiver.
Upon acquiring the satellite signal, the receiver demodulates the SDM from that received signal. The SDM includes satellite ephemeris information concerning the orbits and positions of each satellite, and satellite time model information concerning the satellites"" clocks. A receiver uses this information in combination with the measured pseudo-ranges to determine the receiver""s position by calculating a navigation solution.
The SDM is 900 bits long and is broadcast every 30 seconds at 50 bits/second. Accordingly, a GPS receiver can receive and store the SDM if it is able to receive a satellite""s signal continuously for the amount of time the SDM is broadcast, and during the period that the satellite broadcasts the SDM. Also modulated onto the navigation signal at 50 bits/second is the GPS almanac: a 15,000 bit block of coarse ephemeris and time mode data for the entire GPS constellation. The receiver needs this course data to assist in its acquiring a GPS satellite signal. Receiving the GPS almanac can take at least 12 xc2xd minutes of listening to a single GPS satellite to receive the almanac data.
Other methods of delivering the SDM and GPS almanac data are known. For example, U.S. Pat. No. 5,365,450, incorporated by reference herein for all purposes, describes delivering the SDM via a cellular telephone system to shorten the time required to acquire a GPS satellite. There, a person wanting to rapidly acquire a GPS navigation signal uses a cellular telephone network to request the SDM. The cellular network stores the current ephemeris and time models of the GPS satellite constellation in a GPS satellite almanac database. Upon receiving the request the cellular telephone system responds to the request by sending the requested data over an independent data link. The receiver then uses that data to acquire a GPS satellite signal. However, this system relies on the receiver having an unobscured view of the GPS satellites to continuously track the satellites. Such an unobscured view is not always available to a user, particularly a mobile user.
Certain environments can obstruct the view of the sky. A prime example is the so-called xe2x80x9curban canyonxe2x80x9d where tall buildings densely concentrated in an urban setting obscure significant portions of the sky and block the GPS satellite signals preventing their reception. One way to track GPS satellites in an obscured environment is to use a vector delay lock loop described in U.S. Pat. No. 5,398,034, incorporated by reference herein for all purposes. A vector DLL improves over a conventional delay lock loop by using the receiver""s estimated position vector (x, y, z and time) to control the local PN code generator, rather then using the correlator output as in a conventional DLL circuit. The vector DLL can use measurements from a variety of sources in computing the receiver""s position vector. These sources could include signals from all GPS satellites visible to the receiver, signals from other visible navigation satellites such as satellites in the former Soviet Union""s GLONASS system, and even signals from ground-based navigation transmitters such as so-called xe2x80x9cpseudolitesxe2x80x9d that transmit a GPS-like navigation signal from a known, fixed location. A conventional vector DLL is shown in FIG. 4, and is similar to the receiver shown in FIG. 2, but also contains a H(x) transformation unit 70 that transforms the estimated position information into control signals for controlling the DLLs.
Other techniques for providing navigation signals in environments with obstructed visibility of the sky include embedding navigation beacons in communication signals within a cellular telephone system base station, as described in U.S. Pat. No. 5,604,765, incorporated by reference herein for all purposes. There, the navigation signals are transmitted from a base station either in a standalone or overlay mode where the signal level of the navigation signals is kept below the tolerable noise level of the communications signals. Included in the navigation signal is information identifying the base station and the base station""s location (e.g., base station latitude and longitude). U.S. Pat. No. 6,111,538, incorporated by reference herein for all purposes, describes how a set of spread spectrum navigation beacons, such as those described above, can be designed and arranged in a cellular pattern, and how the required navigation receiver signal processing can be efficiently integrated into a cellular telephone.
A mobile unit equipped with a GPS receiver having a vector DLL must receive updates to the satellite data messages transmitted by the satellites that the receiver is tracking in order to generate accurate position estimates. When the mobile unit is traveling where visibility to the open sky is intermittent at best, it may be able to receive a satellite signal only for brief periods. The vector DLL circuit allows the receiver to continue to track a satellite even during brief interruptions to the signal due to the signal being obscured. However, if the signal is received intermittently for only brief periods, the receiver may not be able to retrieve the SDM during those brief periods when it receives the GPS signal. Accordingly, the receiver may not be able to acquire the information it needs from the satellite data message, or it may have a satellite message that is stale and in need of being refreshed. The receiver also may not have enough storage to hold the satellite data messages for all satellites being tracked. Accordingly, there is a need to provide a GPS receiver with current satellite data messages and GPS almanac data for use with a vector delay lock loop, regardless of the amount of time a GPS signal is available to the receiver. Such capabilities are needed for the vector delay lock loop to operate more accurately and with fresher data than when operating with conventional systems.
Therefore, in light of the above, and for other reasons that will become apparent when the invention is fully described, an object of the invention is to enable a receiver to obtain information concerning the location of a source of a navigation signal even when the receiver is not receiving the navigation signal, and track that source even when the navigation signal is only intermittently received.
Another object of the invention is to use the information concerning the location of the navigation beacon with a vector delay lock loop circuit to enable the receiver to continue to accurately track the navigation beacon when the navigation beacon is received intermittently. In a system in which a mobile terminal may receive multiple navigation signals from different sources, including GPS and signals transmitted from cellular base stations, it is important to have an algorithm that is robust in that it computes a best estimate of position based upon the available signals. In addition, since in many terrestrial environments the signals are intermittent due to blockage, it is important to have an algorithm in which all navigation signals are cooperatively tracked. As a result, the estimate of the mobile terminal""s position is more robust, as long as the minimum number (e.g., four) of navigation signals is received at least intermittently, because blockage of the navigation signal does not prevent the circuit from obtaining the position information of the satellite and terrestrial navigation sources.
Yet another object of the invention is to transmit data concerning the position of a source of a navigation signal, such as a GPS satellite, using a common broadcast control channel of a cellular telephone network. The data is for use by a vector delay lock loop circuit in tracking the navigation signal when it is only intermittently received. The data is also used to allow for rapid reacquisition of a formerly blocked navigation signal by enabling the vector delay lock loop circuit to maintain a running estimate of the signal delay even for blocked signals.
Still another object of the invention is to enable a mobile terminal to use a vector delay lock loop circuit to determine its position from a set of signals that may be transmitted by earth orbiting satellites, such as GPS satellites, or by terrestrial based cellular base stations, or a combination of satellite and terrestrial based stations, in which information concerning the position of the satellites or terrestrial stations is obtained from a cellular common broadcast channel.
Still another object of the invention is to use the communication signals in a cellular telephone network as a navigation signal. According to one aspect of the invention this can be accomplished by receiving a GPS signal at a cellular base station, and using the GPS signal to provide frequency and time references for controlling the characteristics of the cellular communications signals.